Edge-on X-ray detector

ABSTRACT

There is provided an edge-on x-ray detector configured for detecting incoming x-rays. The edge-on x-ray detector includes a plurality of adjacent x-ray sensors, and each x-ray sensor is oriented edge-on to incoming x-rays. The x-ray sensors are arranged side-by-side and/or lined up one after the other, and the interspacing between the x-ray sensors is at least partly filled with a gap filling material including a mixture or compound of resin and metal disulfide.

The project leading to this application has received funding from theEuropean Union's Horizon 2020 research and innovation programme undergrant agreement No 830294.

TECHNICAL FIELD

The proposed technology relates to x-ray imaging, and more particularlyto x-ray detectors as well as x-ray imaging systems.

BACKGROUND

Radiographic imaging such as x-ray imaging has been used for years inmedical applications and for non-destructive testing.

Normally, an x-ray imaging system includes an x-ray source and an x-raydetector system. The x-ray source emits x-rays, which pass through asubject or object to be imaged and are then registered by the x-raydetector system. Since some materials absorb a larger fraction of thex-rays than others, an image is formed of the subject or object.

In order to increase the absorption efficiency, the x-ray detector maybe arranged edge-on to the incoming or incident x-rays, in which casethe absorption depth can be chosen to any length and the detector canstill be fully depleted without going to very high voltages.

Edge-on is thus a special design, where the x-ray sensors are orientededge-on to incoming x-rays. This type of detector may, by design, havex-ray sensors and/or detector elements with a so-called high aspectratio, i.e. a relatively high ratio between the length (or depth) of thex-ray sensors and/or detector elements in the direction of the incidentx-rays and the width of the x-ray sensors and/or detector elements in asubstantially perpendicular direction.

Although edge-on x-ray detectors have many advantageous features, adrawback with high aspect x-ray sensors and/or detector elements thereofis the sensitivity to dynamic misalignments with respect to the focalspot of the x-ray source. Dynamic misalignment is a technical reality,which is non-trivial to eliminate. The effects of dynamic misalignmentmay be clinically unacceptable image artefacts and/or other imagequality problems.

There is thus a need for a technical solution mitigating the effects ofdynamic alignments in edge-on x-ray detectors.

SUMMARY

It is a general object to provide improvements related to x-raydetectors and/or x-ray imaging systems.

For example, it is desirable for an x-ray detector to be more robust todynamic misalignment or to provide a technical solution that at leastreduces the sensitivity to such dynamic misalignments.

It is a specific object to provide an edge-on x-ray detector configuredfor detecting incoming x-rays.

It is also an object to provide an x-ray imaging system comprising suchan x-ray detector.

These and other objects may be achieved by one or more embodiments ofthe proposed technology.

According to a first aspect, there is provided an edge-on x-ray detectorconfigured for detecting incoming x-rays. The edge-on x-ray detectorcomprises a plurality of adjacent x-ray sensors, wherein each x-raysensor is oriented edge-on to incoming x-rays. The x-ray sensors arearranged side-by-side and/or lined up one after the other, and theinterspacing between the x-ray sensors is at least partly filled with agap filling material comprising a mixture or compound of resin and metaldisulfide.

By way of example, although various metal disulfides may be used, it hasbeen shown that a beneficial choice of metal disulfide for the gapfilling mixture or compound is tungsten disulfide. For example, theresin may be mixed with tungsten disulfide powder.

In this way, the edge-on x-ray detector may become more robust todynamic misalignments. In particular, so-called high aspect ratio x-raysensors and/or detector elements may be less sensitive to dynamicmisalignments with respect to the focal point of the x-ray source. Thisin turn may lead to improved image quality. In particular, it may bepossible to more or less eliminate certain image artefacts, e.g. inclinical Computed Tomography (CT) applications.

According to a second aspect, there is provided an x-ray imaging systemincluding such an edge-on x-ray detector.

Expressed slightly differently, according to a third aspect, theproposed technology provides an edge-on x-ray detector configured fordetecting incoming x-rays, which comprises a plurality of adjacent x-raydetector sub-modules, wherein each x-ray detector sub-module is orientededge-on to incoming x-rays. The x-ray detector sub-modules are arrangedside-by-side and/or lined up one after the other, and the interspacingbetween the x-ray detector sub-modules is at least partly filled with amaterial comprising a metal disulfide mixed into a synthetic or organicresin or compound.

Other advantages will be appreciated when reading the detaileddescription.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating an example of an overallx-ray imaging system.

FIG. 2 is a schematic diagram illustrating another example of an x-rayimaging system.

FIG. 3 is a schematic block diagram of a CT system as an illustrativeexample of an X-ray imaging system.

FIG. 4 is a schematic diagram illustrating an example of the conceptualstructure for implementing an energy-discriminating photon-countingdetector.

FIG. 5 is a schematic diagram illustrating an example of a semiconductordetector sub-module according to an exemplary embodiment.

FIG. 6 is a schematic diagram illustrating an example of semiconductordetector sub-module according to another exemplary embodiment.

FIG. 7 is a schematic diagram illustrating an example of an x-raydetector comprising a set of tiled x-ray sensors or detectorsub-modules.

FIG. 8 is a schematic diagram illustrating an example of misalignment ofthe central axis of an x-ray sensor and/or detector element relative thefocal spot of an associated x-ray source.

FIG. 9 is a schematic curve diagram illustrating an example of asituation where more photons would interact in a misaligned x-ray sensorand/or detector element compared to a non-misaligned case.

FIG. 10 is a schematic curve diagram illustrating an example of therelative difference as a function of energy, showing a situation wherethe spectrum of interacting photons is shifted towards the low energyside.

FIG. 11 is a schematic perspective diagram illustrating an example of anx-ray detector comprising a set of adjacent x-ray sensors arrangedside-by-side and/or lined up one after the other.

FIG. 12 is a schematic cross-section diagram illustrating an example ofan x-ray detector comprising a set of adjacent x-ray sensors arrangedside-by-side and/or lined up one after the other.

FIG. 13 is a schematic diagram illustrating an example of an x-raysensor in which individual detector elements could be defined by chargecollection electrodes on the back side.

FIG. 14A is a schematic diagram illustrating an example of a particularimplementation of a detector module based on two x-ray sensors.

FIG. 14B is a schematic diagram illustrating an example of a particularimplementation of an x-ray detector built from several detector modulesof FIG. 14A.

FIG. 15A is a schematic planar diagram illustrating an example of howthe resin-based mixture or compound can be applied on the surface of anx-ray sensor.

FIG. 15B is a schematic cross-section diagram illustrating an example ofa detector module based on two x-ray sensors having a scatter rejectionfoil arranged in-between and with a gap filling material comprising aresin-based mixture or compound of the present invention.

FIG. 16 is a schematic planar diagram illustrating another example ofhow the resin-based mixture or compound can be applied on the surface ofan x-ray sensor.

FIG. 17 is a schematic curve diagram illustrating an example of therelative difference as a function of energy for a misaligned case wheresimply an epoxy glue is used in the interspacing between x-ray sensors.

FIG. 18 is a schematic curve diagram illustrating a first example of therelative difference as a function of energy for a misaligned case wherea material comprising a mixture or compound of an epoxy resin andtungsten disulfide is used in the interspacing between x-ray sensors.

FIG. 19 is a schematic curve diagram illustrating a second example ofthe relative difference as a function of energy for a misaligned casewhere a material comprising a mixture or compound of an epoxy resin andtungsten disulfide is used in the interspacing between x-ray sensors.

FIG. 20 is a schematic diagram illustrating an example of a computerimplementation according to an embodiment.

DETAILED DESCRIPTION

It may be useful to begin with a brief overview of an illustrativeoverall x-ray imaging system, with reference to FIG. 1. In thisnon-limiting example, the x-ray imaging system 100 basically comprisesan x-ray source 10, an x-ray detector system 20 and an associated imageprocessing device 30. In general, the x-ray detector system 20 isconfigured for registering radiation from the x-ray source 10 that mayhave been focused by optional x-ray optics and passed an object orsubject or part thereof. The x-ray detector system 20 is connectable tothe image processing device 30 via suitable analog processing andread-out electronics (which may be integrated in the x-ray detectorsystem 20) to enable image processing and/or image reconstruction by theimage processing device 30.

FIG. 2 is a schematic diagram illustrating an example of an x-rayimaging system 100 comprises an x-ray source 10, which emits x-rays; anx-ray detector system 20, which detects the x-rays after they havepassed through the object; analog processing circuitry 25, whichprocesses the raw electrical signal from the detector and digitizes it;digital processing circuitry 40 which may carry out further processingoperations on the measured data such as applying corrections, storing ittemporarily, or filtering; and a computer 50 which stores the processeddata and may perform further post-processing and/or imagereconstruction.

The overall detector may be regarded as the x-ray detector system 20, orthe x-ray detector system 20 combined with the associated analogprocessing circuitry 25.

The digital part including the digital processing circuitry 40 and/orthe computer 50 may be regarded as a digital image processing system 30,which performs image reconstruction based on the image data from thex-ray detector. The image processing system 30 may thus be seen as thecomputer 50, or alternatively the combined system of the digitalprocessing circuitry 40 and the computer 50, or possibly the digitalprocessing circuitry 40 by itself if the digital processing circuitry isfurther specialized also for image processing and/or reconstruction.

An example of a commonly used x-ray imaging system is a ComputedTomography (CT) system, which may include an x-ray source that producesa fan or cone beam of x-rays and an opposing x-ray detector system forregistering the fraction of x-rays that are transmitted through apatient or object. The x-ray source and detector system are normallymounted in a gantry that rotates around the imaged object.

Accordingly, the x-ray source 10 and the x-ray detector system 20illustrated in FIG. 1 and FIG. 2 may thus be arranged as part of a CTsystem, e.g. mountable in a CT gantry.

FIG. 3 is a schematic block diagram of a CT system as an illustrativeexample of an x-ray imaging system. The CT system comprises a computer50 receiving commands and scanning parameters from an operator via anoperator console 60 that may have has a display and some form ofoperator interface, e.g., keyboard and mouse. The operator suppliedcommands and parameters are then used by the computer 50 to providecontrol signals to an x-ray controller 41, a gantry controller 42 and atable controller 43. To be specific, the x-ray controller 41 providespower and timing signals to the x-ray source 10 to control emission ofx-rays onto the object or patient lying on the table 12. The gantrycontroller 42 controls the rotational speed and position of the gantry11 comprising the x-ray source 10 and the x-ray detector 20. The tablecontroller 43 controls and determines the position of the patient table12 and the scanning coverage of the patient. There is also a detectorcontroller 44, which is configured for controlling and/or receiving datafrom the detector 20.

In an embodiment, the computer 50 also performs post-processing andimage reconstruction of the image data output from the x-ray detector.The computer thereby corresponds to the image processing system 30 asshown in FIGS. 1 and 2. The associated display allows the operator toobserve the reconstructed images and other data from the computer.

The x-ray source 10 arranged in the gantry 11 emits x-rays. An x-raydetector 20, e.g. in the form of an edge-on x-ray detector, detects thex-rays after they have passed through the patient. The x-ray detector 20may for example have a plurality of pixels, also referred to as sensorsor detector elements, and the associated processing circuitry, such asASICs, arranged in detector modules. At least a portion of the analogprocessing part may be implemented in the pixels, whereas any remainingprocessing part is implemented in, for instance, the ASICs. In anembodiment, the processing circuitry (ASICs) digitizes the analogsignals from the pixels. The processing circuitry (ASICs) may alsocomprise a digital processing part, which may carry out furtherprocessing operations on the measured data, such as applyingcorrections, storing it temporarily, and/or filtering. During a scan toacquire x-ray projection data, the gantry and the components mountedthereon rotate about an iso-center.

A challenge for x-ray imaging detectors is to extract maximuminformation from the detected x-rays to provide input to an image of anobject or subject where the object or subject is depicted in terms ofdensity, composition and structure. It is still common to usefilm-screen as detector but most commonly the detectors today provide adigital image.

Modern x-ray detectors normally need to convert the incident x-rays intoelectrons, this typically takes place through photo absorption orthrough Compton interaction and the resulting electrons are usuallycreating secondary visible light until its energy is lost and this lightis in turn detected by a photo-sensitive material. There are alsodetectors, which are based on semiconductors and in this case theelectrons created by the x-ray are creating electric charge in terms ofelectron-hole pairs which are collected through an applied electricfield.

Conventional x-ray detectors are energy integrating, the contributionfrom each detected photon to the detected signal is thereforeproportional to its energy, and in conventional CT, measurements areacquired for a single energy distribution. The images produced by aconventional CT system therefore have a certain look, where differenttissues and materials show typical values in certain ranges.

There are detectors operating in an integrating mode in the sense thatthey provide an integrated signal from a multitude of x-rays and thesignal is only later digitized to retrieve a best guess of the number ofincident x-rays in a pixel.

However, photon counting detectors have emerged as a feasiblealternative in some applications; currently those detectors arecommercially available mainly in mammography. The photon countingdetectors have an advantage since in principle the energy for each x-raycan be measured which yields additional information about thecomposition of the object. This information can be used to increase theimage quality and/or to decrease the radiation dose.

A further improvement relates to the development of so-calledenergy-discriminating photon-counting detectors, e.g. as schematicallyillustrated in FIG. 4. In this type of x-ray detectors, each registeredphoton generates a current pulse which is compared to a set ofthresholds, thereby counting the number of photons incident in each of anumber of so-called energy bins. This may be very useful in the imagereconstruction process. Sometimes, an energy-discriminatingphoton-counting detector may be referred to as a multi-bin detector.

In general, the energy information allows for new kinds of images to becreated, where new information is available and image artifacts inherentto conventional technology can be removed.

In other words, for an energy-discriminating detector, the pulse heightsare compared to a number of programmable thresholds in the comparatorsand classified according to pulse-height, which in turn is proportionalto energy.

However, an inherent problem in any (charge sensitive) amplifier is thatit will add electronic noise to the detected current. In order to avoiddetecting noise instead of real x-ray photons, it is therefore importantto set the lowest threshold value (Thr) high enough so that the numberof times the noise value exceeds the threshold value is low enough notto disturb the detection of x-ray photons.

By setting the lowest threshold above the noise floor, electronic noise,which is the major obstacle in the reduction of radiation dose of thex-ray imaging systems, can be significantly reduced.

The (shaping) filter has the general property that large values of theshaping time will lead to a long pulse caused by the x-ray photon andreduce the noise amplitude after the filter. Small values of the shapingtime will lead to a short pulse and a larger noise amplitude. Therefore,in order to count as many x-ray photons as possible, a large shapingtime is desired to minimize noise and allowing the use of a relativelysmall threshold level.

As previously mentioned, in order to increase the absorption efficiency,the x-ray detector can be arranged edge-on, in which case the absorptiondepth can be chosen to any length and the detector can still be fullydepleted without going to very high voltages.

Edge-on is thus a special design, where the x-ray sensors are orientededge-on to incoming x-rays.

For example, such an edge-on x-ray detector may have pixels or detectorelements in at least two directions, wherein one of the directions ofthe edge-on detector has a component in the direction of the X-rays.Such an edge-on x-ray detector is sometimes referred to as adepth-segmented x-ray detector, having two or more depth segments ofdetector elements in the direction of the incoming X-rays.

Alternatively, the detector elements may be arranged as an array(non-depth-segmented) in a direction substantially orthogonal to thedirection of the incident x-rays, and each of the detector elements maybe oriented edge-on to the incident x-rays. In other words, the x-raydetector may be non-depth-segmented, while still arranged edge-on to theincoming x-rays.

FIG. 5 is a schematic diagram illustrating an example of an edge-onx-ray sensor 21, also referred to as a detector module or sub-module,according to an exemplary embodiment. This is an example of an x-raysensor 21 or detector sub-module with the sensor part split intodetector elements or pixels, where each detector element (or pixel) isnormally based on a diode having a charge collecting electrode as a keycomponent. The x-rays enter through the edge of the x-ray sensor.

FIG. 6 is a schematic diagram illustrating an example of an edge-onx-ray sensor 21, or detector sub-module, according to another exemplaryembodiment. In this example, the sensor part is further split intoso-called depth segments in the depth direction, again assuming thex-rays enter through the edge.

Normally, a detector element is an individual x-ray sensitivesub-element of the detector. In general, the photon interaction takesplace in a detector element and the thus generated charge is collectedby the corresponding electrode of the detector element.

Each detector element typically measures the incident x-ray flux as asequence of frames. A frame is the measured data during a specified timeinterval, called frame time.

Depending on the detector topology, a detector element may correspond toa pixel, especially when the detector is a flat-panel detector. Adepth-segmented detector may be regarded as having a number of detectorstrips, each strip having a number of depth segments. For such adepth-segmented detector, each depth segment may also be regarded as anindividual detector element, especially if each of the depth segments isassociated with its own individual charge collecting electrode.

The detector strips of a depth-segmented detector normally correspond tothe pixels of an ordinary flat-panel detector, and therefore sometimesalso referred to as pixel strips. However, it is also possible to regarda depth-segmented detector as a three-dimensional pixel array, whereeach pixel (sometimes referred to as a voxel) corresponds to anindividual depth segment/detector element.

By way of example, the sensors may be implemented as so calledMulti-Chip Modules (MCMs) in the sense that the semiconductor sensorsare used as base substrates for electric routing and for a number ofApplication Specific Integrated Circuits (ASICs) which are attachedpreferably through so called flip-chip technique. The routing willinclude a connection for the signal from each pixel or detector elementto the ASIC input as well as connections from the ASIC to externalmemory and/or digital data processing. Power to the ASICs may beprovided through similar routing taking into account the increase incross-section which is required for the large currents in theseconnections, but the power may also be provided through a separateconnection.

FIG. 7 is a schematic diagram illustrating an example of an x-raydetector 20 comprising a set of tiled x-ray sensors, 21, also referredto as x-ray detector sub-modules, where each x-ray sensor or detectorsub-module 21 is a depth-segmented edge-on x-ray sensor or detectorsub-module and the ASICs or corresponding circuitry 24 are arrangedbelow the detector elements 22 as seen from the direction of theincoming x-rays, allowing for routing paths 23 from the detectorelements 22 to the ASICs 24 in the space between detector elements.

As previously mentioned, a drawback with high aspect detector elementssuch as those of edge-on x-ray detectors is the sensitivity to dynamicmisalignments with respect to the focal spot of the x-ray source.Dynamic misalignment is a technical reality, which is non-trivial toeliminate. The effects of dynamic misalignment may be clinicallyunacceptable image artefacts and/or other image quality problems.

There is thus a need for a technical solution mitigating the effects ofdynamic alignments in edge-on x-ray detectors.

For a better understanding, an illustrative problem scenario will bediscussed in more detail below.

By way of example, photon counting multi-bin detectors have thecapability to partition the incident photons of a broad spectrum intoseveral bins based on their deposited energy. This is typically done bypulse-height comparators (e.g. once again see FIG. 3) acting on ananalogue signal with an amplitude proportional to the charge liberatedby the x-ray in the semi-conductor material of the detector.

Benefits of using photon-counting multi-bin detectors in ComputedTomography applications include elimination of beam hardening artifacts,material quantification and the possibility to generate images withimproved contrast to noise ratio by means of generating syntheticmonoenergetic images. These benefits are typically obtained by materialbasis decomposition. Often material basis decomposition is performed inthe projection domain i.e. applied to the raw photon counts in everysingle projection. In short, the goal for such a decomposition is to usethe counts in each projection and estimate the corresponding pathlengthsof a set of basis materials. These pathlengths estimate are later usedfor image reconstruction.

A pre-requisite for performing material basis decomposition is accurateknowledge of the system. This knowledge is typically captured in aforward model, describing the expected response (counts in each bin) forall feasible combinations of basis material pathlengths. Material basisdecomposition amounts to inverting the forward model using noisy x-rayrealizations (detected counts in the bins) and estimated correspondingpathlengths. One possible method to do this is to use the maximumlikelihood method.

The larger the aspect ratio of individual detector elements, i.e. ratiobetween the length in the direction of the incident x-rays and thelength in a perpendicular direction, the more sensitive the detectorbecomes to dynamic misalignment of the detector elements and the focalspot source.

With dynamic misalignment we refer to a displacement of the center axisof an x-ray sensor and/or detector element relative the focal spotsource, cf. FIG. 8. There are many sources for such movements, includingthermal expansion, rotational forces and focal spot movement and theywill be elaborated on below. It is important to already here comment onthe possible effect of such movements; if severe, they can make theforward model invalid. If, for example, the calibration data for acertain x-ray sensor and/or detector element was obtained with thesensor axis pointing towards the focal spot, the determined forwardmodel will only be exact for this alignment. If misalignment causesx-rays to enter an x-ray sensor and/or detector element with largeaspect ratio at an angle, both the number of photons interacting and theenergies they deposit may differ since the attenuation lengths throughthe active detector material vary greatly for x-rays that hit the x-raysensor and/or detector element at the slanted edge. While x-rays alsohit a misaligned x-ray sensor and/or detector element with small aspectratio at the slanted edge, the fraction of x-rays hitting the edgeincreases with aspect ratio as evident from FIG. 8. If the difference innumber of photons interacting, and their energy, is large enough aforward model determined during correct alignment will not be accurateenough to perform material basis decomposition without bias.

A more thorough understanding as to why the energies of the interactingphotons differ in the two cases is given by the exponential transmissiondescribed by the Beer-Lambert law. Even though the illuminated x-raysensor and/or detector element volumes (cf. FIG. 8) do not change withmisalignment angle, the pathlengths will differ. A larger fraction ofthe x-ray sensor and/or detector element will only generate counts fromx-rays that have traversed a short distance, and therefore morelow-energy photons will interact compared to the case of nomisalignment. Similarly, the fraction the x-rays that traverse adistance corresponding to the full detector length is smaller in thecase of misalignment, and this will result in relatively fewerhigh-energy photons interacting. The effect can be sizeable asillustrated by means of simulation, results of which can be seen in FIG.9. The simulation parameters, although merely intended to beillustrative and non-limiting, are based on a silicon strip detectorwith a width of 650 μm and height of 37 mm, yielding an aspect ratio of57. The misalignment corresponds to an exaggerated tilt angle of 0.115°.It is clear that the spectrum of interacting photons is changed and intotal 3.2% more photons would interact in a misaligned x-ray sensor(dashed line) and/or detector element compared to a non-misaligned case(solid line).

In FIG. 10, the relative difference as a function of energy is depicted.It shows that the spectrum of interacting photons is shifted towards thelow energy side.

Thus far we have shown that dynamic misalignment might cause biasedmaterial pathlength estimates if, for example, the focal spot has movedin the time between calibration and image acquisition. If all individualx-ray sensors and/or detector elements were identical, they would reactthe same way to a dynamic misalignment, i.e. with the same bias.However, there are also unavoidable static misalignments of the x-raysensor and/or detector elements stemming from, for instance,inaccuracies during mounting. Two different statically misaligned x-raysensor and/or detector elements will react differently to the samedynamic misalignment due to the non-linear nature of x-ray attenuation.From this one can conclude that the resulting bias in the material basispathlengths will be different. In third generation CT such differences,if large enough, will generate ring artifacts during reconstructionwhich is clinically intolerable in patient images.

Dynamic misalignment is therefore a real image quality problem and itseffects are aggravated for systems using x-ray sensor and/or detectorelements with large aspect ratio. The sources of dynamic misalignmentare furthermore not simple to eliminate. For example, while the possiblebending of the detector cradle during rotation is predictable and theeffect is therefore possible to calibrate for, calibrating the systemfor all possible rotation speeds is very time consuming in practice.Similarly, even though thermal expansion (from heat generated by thedetector modules during the scan) can be expected to behave predictablewith temperature, calibration at all different possible detectortemperatures (which depend on the ambient surrounding and the scanduration) is also not practical. Finally, dynamic focal spot motion,although efforts have been made to minimize it, is a technical realityin all x-ray tubes with rotating anode used in clinical practice.

This more detailed problem description has served to show that dynamicmisalignment is a technical reality which is non-trivial to eliminate.The effects of dynamic misalignment may be clinically unacceptable imageartifacts, especially for photon counting multi-bin detectors with largeaspect ratio used for CT applications.

It is a general object to provide improvements related to x-raydetectors and/or x-ray imaging systems.

For example, it is desirable for an x-ray detector to be more robust todynamic misalignment or to provide a technical solution that at leastreduces the sensitivity to such dynamic misalignments.

It is a specific object to provide an edge-on x-ray detector configuredfor detecting incoming x-rays.

It is also an object to provide an x-ray imaging system comprising suchan x-ray detector.

These and other objects may be achieved by one or more embodiments ofthe proposed technology.

According to a first aspect, there is provided an edge-on x-ray detectorconfigured for detecting incoming x-rays. The edge-on x-ray detectorcomprises a plurality of adjacent x-ray sensors, wherein each x-raysensor is oriented edge-on to incoming x-rays. The x-ray sensors arearranged side-by-side and/or lined up one after the other, and theinterspacing between the x-ray sensors is at least partly filled with agap filling material comprising a mixture or compound of resin and metaldisulfide.

FIG. 11 is a schematic perspective diagram illustrating an example of anx-ray detector 20 comprising a set of adjacent x-ray sensors 21 arrangedside-by-side and/or lined up one after the other. As can be seen, thex-ray sensors are oriented edge-on to incoming x-rays.

FIG. 12 is a schematic cross-section diagram illustrating an example ofan x-ray detector 20 comprising a set of adjacent x-ray sensors 21arranged side-by-side and/or lined up one after the other, wherein eachx-ray sensor is oriented edge-on to incoming x-rays. The cross-sectiondiagram of FIG. 12 also illustrates how the interspacing is at leastpartially filled with an interspacing/filling material 26. This material26 comprises a mixture or compound of resin and metal disulfide.

FIG. 13 is a schematic diagram illustrating an example of an x-raysensor in which individual detector elements could be defined by chargecollection electrodes on the back side.

Typically, there is a desire for the x-ray sensors 21 to be close toeach other to maximize the active detector area/volume and thus doseefficiency, but at the same time they should not be in direct physicalcontact with each other due to the apparent risk of short circuiting.From a manufacturing perspective, and since the aspect ratio of x-raysensors 21 and/or the detector elements thereof is high, there is a needfor some lateral support and interspacing and/or filling materialbetween the x-ray sensors 21, e.g. to ensure a rigid structure andproper functionality. This interspacing and/or filling material couldfor example be based on a resin (such as glue or another adhesive, or ingeneral, any organic or synthetic resin) which has the benefit oflending itself to automatic dispensing by an industrial robot as part ofthe manufacturing process, allowing for a streamlined productionprocess.

Parasitic capacitance between the x-ray sensors and/or detector elementsaffects the electronic noise level and therefore the performance of thedetector. This parasitic capacitance depends inversely on the distanceseparating two x-ray sensors and also depends on the permittivity(dielectric constant) of the material separating them. Accordingly, froma dose efficiency point of view one would want the x-ray sensors to beas close as possible and from a noise perspective one would want them tobe further apart, to keep parasitic capacitance and thereby noise to aminimum.

It is beneficial for the gap filling material to have x-ray attenuatingproperties. For example, if it may be desirable to provide a mixture orcompound that mimics the attenuation of silicon, especially ifsilicon-based x-ray sensors are used.

More generally, the invention proposes mixing an x-ray attenuating metaldisulfide (e.g. in the form of a powder) into a resin to lower thesensitivity to dynamic misalignments.

In other words, the inventors have found that mixing metal disulfideinto the resin provides a technical solution that supports usefulproperties and features as mentioned above, and as will be furtherelaborated on later on.

In this way, the edge-on x-ray detector may become more robust todynamic misalignments. In particular, so-called high aspect ratio x-raysensors and/or detector elements may be less sensitive to dynamicmisalignments with respect to the focal point of the x-ray source. Thisin turn may lead to improved image quality. In particular, it may bepossible to more or less eliminate certain image artefacts, e.g. inclinical CT applications.

Expressed slightly differently, the assembly process results in somewhatvariable sensor-to-sensor and sensor-to-lamella distances (in thez-direction). If a standard glue is used (with a low average atomicnumber like epoxy) the x-ray attenuation of the glue-filled gaps in theassembly would be very low. This would make the design highly sensitiveto dynamic focal spot movements (in the z-direction) since differentsensors would react very differently. Spectral change induced by thefocal spot shift. This would result in ring artefacts. By using a glueor resin mixture obtained by mixing tungsten disulfide (or any othermetal powder with high atomic number) into the glue or resin at aconcentration that makes the glue-gap appear as silicon-like aspossible, the sensitivity to artefacts will be significantly reduced.

FIG. 14A is a schematic diagram illustrating an example of a particularimplementation of a detector module based on two x-ray sensors. In thisexample, the x-ray sensors 21 arranged side-by-side, having a scatterrejection foil or plate 27 located between the x-ray sensors. The gapbetween the x-ray sensors 21 is further at least partially filled with amaterial 26 based on a mixture or compound of resin and metal disulfide,a so-called resin-based mixture.

FIG. 14B is a schematic diagram illustrating an example of a particularimplementation of an x-ray detector built from several detector modulesof FIG. 14A. Here, it can be seen that the interspacing between detectormodules is also at least partially filled with the resin-based mixture26. It can also be seen that the x-ray sensors 21 (as well as theoverall x-ray detector assembly or x-ray detector) is oriented edge ontoward the incident x-rays, hence defining an edge-on x-ray detector 20.

By way of example, although various metal disulfides may be used, it hasbeen shown that a beneficial choice of metal disulfide for the gapfilling mixture or compound is tungsten disulfide. For example, theresin may be mixed with tungsten disulfide powder. Tungsten disulfide isan inorganic chemical compound composed of tungsten and sulfur with thechemical formula WS2 or WS₂.

In a particular example, the product of the fill factor of the mixtureand the weight fraction of tungsten disulfide in the mixture rangesbetween 5 and 15%.

More desirably, the product of the fill factor of the mixture and theweight fraction of tungsten disulfide in the mixture may be in the rangebetween 6 and 12%.

In a further particular example, it has been shown that reallysatisfactory performance can be achieved when the product of the fillfactor of the mixture and the weight fraction of tungsten disulfide inthe mixture ranges between 8 and 10%.

By way of example, the fill factor of the mixture may be defined as thefraction of interspacing void between x-ray sensors occupied by themixture.

As an example, the resin may be an organic or synthetic resin.

In a particular example, the resin may have adhesive properties, e.g. inthe form of an adhesive and/or glue.

For example, the resin may include an adhesive based on epoxy, acrylate,silicon, polyvinyl acetate and/or urethane or polyurethane.

Optionally, the x-ray detector may further comprise scatter rejectionfoils or plates arranged between at least part of the x-ray sensors.

For example, these scatter rejection foils or plates may be made oftungsten.

By way of example, the x-ray sensors may be edge-on x-ray sensors, eachhaving a number of detector elements, e.g. as illustrated in FIGS. 5-7.

In a particular example, each edge-on x-ray sensor is based on a siliconwafer having a number of detector electrodes.

As schematically illustrated, each edge-on x-ray sensor may includedetector elements extending in the depth direction of the edge-on x-raysensor, assuming x-rays enter through the edge.

In a particular example, each edge-on x-ray sensor is a depth-segmentedx-ray sensor having two or more depth segments of detector elements inthe direction of the incoming x-rays, e.g. see FIG. 6 or FIG. 7.

For example, the x-ray sensors may be arranged side-by-side and/or linedup one after the other in a direction substantially perpendicular to thedirection of the incoming x-rays and/or arranged in a slightly curvedoverall configuration with respect to the x-ray focal point of anassociated x-ray source.

As an example, the x-ray sensors may be planar modules, and, for atleast part of the x-ray sensors, the x-ray sensors may be arrangedside-by-side in the so-called in-plane direction of the x-ray sensorsand/or in a direction transversely to the in-plane direction.

In a particular example, the edge-on x-ray detector may be aphoton-counting x-ray detector, e.g. a photon-counting multi-bin x-raydetector.

The so-called resin-based mixture or compound having a metal disulfidemixed with resin can be applied on the surface of an x-ray sensor inmany different ways, e.g. applied in strings or in a dot pattern on atleast a subset of the adjacent x-ray sensors.

FIG. 15A is a schematic planar diagram illustrating an example of howthe resin-based mixture or compound can be applied on the surface of anx-ray sensor. In this example, the resin-based mixture with metaldisulfide 26 is applied in strings on the planar surface of the x-raysensor substrate. Optionally, an additional adhesive 28 may be applied,preferably in dots to lock and/or strengthen the connection and rigiditybetween adjacent x-ray sensors. This optional adhesive 28 may forexample be a UV and/or heat cured adhesive.

FIG. 15B is a schematic cross-section diagram illustrating an example ofa detector module based on two x-ray sensors having a scatter rejectionfoil arranged in-between and with a gap filling material comprising aresin-based mixture or compound of the present invention.

FIG. 16 is a schematic planar diagram illustrating another example ofhow the resin-based mixture or compound can be applied on the surface ofan x-ray sensor.

As previously mentioned, parasitic capacitance between the x-ray sensorsand/or detector elements affects the electronic noise level andtherefore the performance of the detector. This parasitic capacitancedepends inversely on the distance separating two x-ray sensors and alsodepends on the permittivity (dielectric constant) of the materialseparating them. Accordingly, from a dose efficiency point of view onewould want the x-ray sensors to be as close as possible and from a noiseperspective one would want them to be further apart, to keep parasiticcapacitance and thereby noise to a minimum.

Most resins and/or glues have relatively low average atomic number andare therefore, in terms of the x-ray linear attenuation coefficients,quite similar to air. As described above, this results in a sensitivityto dynamic misalignments between the detector element axes and the focalspot. This is illustrated in FIG. 17, where a simulation is carried outfor a silicon strip detector with a large aspect ratio of 57. To makethe simulation more realistic, the dynamic misalignment corresponds to adynamic focal spot movement of 0.2 mm. Given an assumedsource-to-detector distance of 1 m, this corresponds to a dynamicmisalignment of 0.0115°

$\left( {\tan^{- 1}\left( \frac{0.2}{1000} \right)} \right).$In this example, the void between the x-ray sensors and/or detectorelements is assumed to be filled hardened epoxy glue. In total, 0.14%more photons interact in the detector (compared to the non-misalignedcase) and the ratio of the photon energies is shown in FIG. 17. FromFIG. 17 it can be seen that, for example, a movement of the focal spotof 0.2 mm will result in 2000 ppm more photons in the energy range 60-70keV interacting in the detector and that the difference increases thelower the energy.

The figures 2000 ppm and 0.14% might appear small but for detectors usedfor third generation CT, this will unfortunately result in visible ringartifacts.

It is desirable, e.g. for a CT detector assembly to be robust to dynamicmisalignment.

As already outlined above, the invention proposes mixing an x-rayattenuating metal disulfide (e.g. in the form of a powder) into a resinsuch as epoxy glue to lower the sensitivity to dynamic misalignments.

As a basis for discussing implementation details, the metal powder andits concentration should preferably be selected both to minimize thespectral sensitivity of the detector assembly to dynamic misalignmentsand to keep parasitic capacitance between x-ray sensors and/or detectorelements to a minimum. At the same time desired characteristics of theresin (such as epoxy glue), i.e. non-granularity and a viscosity thatallows it to be automatically dispensed, should preferably bemaintained.

For example, mixing molybdenum disulfide in the resin might result indesirable x-ray attenuation properties but require a high mass fraction,due to the relative low atomic number of molybdenum. In some cases, themixing itself may become difficult or the resin-powder mixture maybecome too viscous. This might be the case particularly if theinterspacing between x-ray sensors and/or detector elements is notcompletely filled with the resin-based mixture or compound, for instanceif the resin is dispensed in lines with a fill factor less than unity.In such a case, the weight fraction of the power must be correspondinglylarger to yield a similar attenuation. Using a fill factor less thanunity is desirable to keep parasitic capacitance to a minimum. Theresin-based mixture must furthermore not be electrically conductive asthis would lead to short circuiting of the detector elements. This makesthe use of industrially available glues that have been made conductiveby addition of metal grains unfeasible.

It may be more advantageous to use a metal disulfide such as tungstendisulfide (WS2), rather than molybdenum disulfide. Selecting tungstendisulfide instead of molybdenum disulfide as a powder to mix into theresin or glue allow a small fraction by weight, owing to the higheratomic number, overcoming the practical problem of obtaining a lumpynon-viscous mixture. WS2 also has low permittivity keeping parasiticcapacitance low and allowing the x-ray sensor spacing to be small.

The spectral sensitivity to dynamic misalignment is however sensitive toconcentration. For example, using the same simulation parameters asabove, and adding 13% WS2 by weight to a resin such as epoxy glue,results in a change of 500 ppm in total counts and an energydistribution like in FIG. 18.

If 9.5% WS2 by weight is added to the resin such as epoxy glue, thetotal number detected photons, regardless of energy, will differ a mere60 ppm when 0.0115° misaligned compared to the non-misaligned case. Thiseffect is negligible. FIG. 19 shows that, for the most prominent x-rayenergies of the spectrum 80-100 keV, the resulting difference from the0.0115° misalignment is in the order of hundreds of parts per million(ppm). This is a level that is tolerable in third generation CT and willin general not result in ring artifacts.

This shows that with careful considerations, a so-called high aspectratio x-ray detector can be made more or less insensitive to dynamicmisalignments using the proposed method.

According to a second aspect, there is provided an x-ray imaging systemincluding such an edge-on x-ray detector.

Expressed slightly differently, according to a third aspect, theproposed technology provides an edge-on x-ray detector configured fordetecting incoming x-rays, which comprises a plurality of adjacent x-raydetector sub-modules, wherein each x-ray detector sub-module is orientededge-on to incoming x-rays. The x-ray detector sub-modules are arrangedside-by-side and/or lined up one after the other, and the interspacingbetween the x-ray detector sub-modules is at least partly filled with amaterial comprising a metal disulfide mixed into a synthetic or organicresin or compound.

It will be appreciated that the methods and devices described herein canbe combined and re-arranged in a variety of ways.

For example, specific functions may be implemented in hardware, or insoftware for execution by suitable processing circuitry, or acombination thereof.

The steps, functions, procedures, modules and/or blocks described hereinmay be implemented in hardware using any conventional technology, suchas semiconductor technology, discrete circuit or integrated circuittechnology, including both general-purpose electronic circuitry andapplication-specific circuitry.

Particular examples include one or more suitably configured digitalsignal processors and other known electronic circuits, e.g. discretelogic gates interconnected to perform a specialized function, orApplication Specific Integrated Circuits (ASICs).

Alternatively, at least some of the steps, functions, procedures,modules and/or blocks described herein may be implemented in softwaresuch as a computer program for execution by suitable processingcircuitry such as one or more processors or processing units.

Examples of processing circuitry includes, but is not limited to, one ormore microprocessors, one or more Digital Signal Processors (DSPs), oneor more Central Processing Units (CPUs), video acceleration hardware,and/or any suitable programmable logic circuitry such as one or moreField Programmable Gate Arrays (FPGAs), or one or more ProgrammableLogic Controllers (PLCs).

It should also be understood that it may be possible to re-use thegeneral processing capabilities of any conventional device or unit inwhich the proposed technology is implemented. It may also be possible tore-use existing software, e.g. by reprogramming of the existing softwareor by adding new software components.

FIG. 20 is a schematic diagram illustrating an example of a computerimplementation according to an embodiment. In this particular example,the system 200 comprises a processor 210 and a memory 220, the memorycomprising instructions executable by the processor, whereby theprocessor is operative to perform the steps and/or actions describedherein. The instructions are typically organized as a computer program225; 235, which may be preconfigured in the memory 220 or downloadedfrom an external memory device 230. Optionally, the system 200 comprisesan input/output interface 240 that may be interconnected to theprocessor(s) 210 and/or the memory 220 to enable input and/or output ofrelevant data such as input parameter(s) and/or resulting outputparameter(s).

The term ‘processor’ should be interpreted in a general sense as anysystem or device capable of executing program code or computer programinstructions to perform a particular processing, determining orcomputing task.

The processing circuitry including one or more processors is thusconfigured to perform, when executing the computer program, well-definedprocessing tasks such as those described herein.

The processing circuitry does not have to be dedicated to only executethe above-described steps, functions, procedure and/or blocks, but mayalso execute other tasks.

The proposed technology also provides a computer-program productcomprising a computer-readable medium 220; 230 having stored thereonsuch a computer program.

By way of example, the software or computer program 225; 235 may berealized as a computer program product, which is normally carried orstored on a computer-readable medium 220; 230, in particular anon-volatile medium. The computer-readable medium may include one ormore removable or non-removable memory devices including, but notlimited to a Read-Only Memory (ROM), a Random Access Memory (RAM), aCompact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray disc, aUniversal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storagedevice, a flash memory, a magnetic tape, or any other conventionalmemory device. The computer program may thus be loaded into theoperating memory of a computer or equivalent processing device forexecution by the processing circuitry thereof.

Method flows may be regarded as a computer action flows, when performedby one or more processors. A corresponding device, system and/orapparatus may be defined as a group of function modules, where each stepperformed by the processor corresponds to a function module. In thiscase, the function modules are implemented as a computer program runningon the processor. Hence, the device, system and/or apparatus mayalternatively be defined as a group of function modules, where thefunction modules are implemented as a computer program running on atleast one processor.

The computer program residing in memory may thus be organized asappropriate function modules configured to perform, when executed by theprocessor, at least part of the steps and/or tasks described herein.

Alternatively, it is possibly to realize the modules predominantly byhardware modules, or alternatively by hardware. The extent of softwareversus hardware is purely implementation selection.

The embodiments described above are merely given as examples, and itshould be understood that the proposed technology is not limitedthereto. It will be understood by those skilled in the art that variousmodifications, combinations and changes may be made to the embodimentswithout departing from the present scope as defined by the appendedclaims. In particular, different part solutions in the differentembodiments can be combined in other configurations, where technicallypossible.

The invention claimed is:
 1. An edge-on x-ray detector configured fordetecting incoming x-rays, said edge-on x-ray detector comprising: aplurality of adjacent x-ray sensors, each of the x-ray sensors beingoriented edge-on to incoming x-rays, the x-ray sensors being arrangedside-by-side and/or lined up one after the other, the interspacingbetween the x-ray sensors being at least partly filled with a gapfilling material comprising a mixture or compound of resin and tungstendisulfide, wherein the product of the fill factor of the mixture and theweight fraction of tungsten disulfide in the mixture ranges between 5and 15%.
 2. The edge-on x-ray detector of claim 1, wherein the resin ismixed with tungsten disulfide powder.
 3. The edge-on x-ray detector ofclaim 1, wherein the product of the fill factor of the mixture and theweight fraction of tungsten disulfide in the mixture ranges between 8and 10%.
 4. The edge-on x-ray detector of claim 1, wherein the fillfactor of the mixture is defined as the fraction of interspacing voidbetween x-ray sensors occupied by the mixture.
 5. The edge-on x-raydetector of claim 1, wherein the resin is an organic or synthetic resin.6. The edge-on x-ray detector of claim 1, wherein the resin has adhesiveproperties.
 7. The edge-on x-ray detector of claim 1, wherein the resinincludes an adhesive based on epoxy, acrylate, silicon, polyvinylacetate and/or urethane or polyurethane.
 8. The edge-on x-ray detectorof claim 1, further comprising scatter rejection foils or platesarranged between at least part of the x-ray sensors.
 9. The edge-onx-ray detector of claim 8, wherein the scatter rejection foils or platesare made of tungsten.
 10. The edge-on x-ray detector of claim 1, whereinthe x-ray sensors are edge-on x-ray sensors, each having a number ofdetector elements.
 11. The edge-on x-ray detector of claim 10, whereineach edge-on x-ray sensor is based on a silicon wafer having a number ofdetector electrodes.
 12. The edge-on x-ray detector of claim 10, whereineach edge-on x-ray sensor comprises detector elements extending in thedepth direction of the edge-on x-ray sensor, assuming x-rays enterthrough the edge.
 13. The edge-on x-ray detector of claim 10, whereineach edge-on x-ray sensor is a depth-segmented x-ray sensor having twoor more depth segments of detector elements in the direction of theincoming x-rays.
 14. The edge-on x-ray detector of claim 1, wherein thex-ray sensors are arranged side-by-side and/or lined up one after theother in a direction substantially perpendicular to the direction of theincoming x-rays and/or arranged in a slightly curved overallconfiguration with respect to the x-ray focal point of an associatedx-ray source.
 15. The edge-on x-ray detector of claim 1, wherein thex-ray sensors are planar modules, and, for at least part of the x-raysensors, the x-ray sensors are arranged side-by-side in the in-planedirection of the x-ray sensors and/or in a direction transversely to thein-plane direction.
 16. The edge-on x-ray detector of claim 1, whereinthe edge-on x-ray detector is a photon-counting x-ray detector.
 17. Theedge-on x-ray detector of claim 1, wherein the gap filling material isapplied in strings or in a dot pattern on at least a subset of theadjacent x-ray sensors.
 18. An x-ray imaging system comprising: theedge-on x-ray detector of claim
 1. 19. An edge-on x-ray detectorconfigured for detecting incoming x-rays, said edge-on x-ray detectorcomprising: a plurality of adjacent x-ray sensors, each of the x-raysensors being oriented edge-on to incoming x-rays, the x-ray sensorsbeing arranged side-by-side and/or lined up one after the other, theinterspacing between the x-ray sensors being at least partly filled witha gap filling material comprising a mixture or compound of resin andtungsten disulfide, wherein the product of the fill factor of themixture and the weight fraction of tungsten disulfide in the mixtureranges between 6 and 12%.
 20. An edge-on x-ray detector configured fordetecting incoming x-rays, wherein said x-ray detector comprises aplurality of adjacent x-ray detector sub-modules, wherein each x-raydetector sub-module is oriented edge-on to incoming x-rays, wherein thex-ray detector sub-modules are arranged side-by-side and/or lined up oneafter the other, and the interspacing between the x-ray detectorsub-modules is at least partly filled with a material comprising a metaldisulfide mixed into a synthetic or organic resin or compound.